Lithium secondary batteries are small, lightweight, high-capacity batteries which have been widely used since appearing in 1991. In particular, as recent, rapid advancements in the electronics, communications, and computer industries have led to the development of camcorders, mobile phones, notebook computers, and the like, demand for lithium secondary batteries as a power source for driving these mobile electronic data communication devices is continuously increasing.
Lithium secondary batteries are manufactured by charging an organic electrolyte solution or polymer electrolyte solution between a positive electrode and negative electrode in which lithium ions may be reversibly intercalated and deintercalated, and electrical energy is generated by a redox reaction which occurs when the lithium ion is intercalated/deintercalated in the positive electrode or the negative electrode.
Currently, chalcogenide compounds are used for the positive electrode, and examples thereof include transition metal composite oxides such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-x1Cox1O2(0<x1<1), and LiMnO2, which are currently being developed.
Moreover, in addition to lithium metal, materials with high theoretical capacity are being used as negative electrode active materials, for example, crystalline carbons such as graphite and synthetic graphite, and amorphous carbons such as soft carbon and hard carbon, and the like. However, although the crystalline carbon has a relatively high capacity, the capacity is still only 380 mAh/g, and thus it will be difficult to be used in the development of future high-capacity lithium secondary batteries. Moreover, although the amorphous carbon has a high theoretical capacity, the amorphous carbon exhibits a high non-reversibility during the charging/discharging process. When the non-reversible capacity is high, the lithium ions are consumed, making it is impossible to completely charge/discharge the active material, and thus have a disadvantageous effect on the energy density of the battery.
In order to overcome these limitations, a method has been researched in which a metal-based or intermetallic compound-based materials such as on aluminum, germanium, silicon (Si), tin (Sn), zinc, and lead, is used as negative electrode active materials in place of carbon-based materials as above.
The metal-based or intermetallic compound-based materials have high capacity and high energy densities, and are able to store and release more lithium ions than carbon-based materials, and thus are desirable for manufacturing secondary batteries having high capacities and high energy densities.
However, the metal-based or intermetallic compound-based materials have worse cycle characteristics than carbon-based materials, and thus have been difficult to commercialize. For example, when inorganic particles such as the silicon or tin are used as-is as negative electrode active materials, excessive volume changes that occur due to lithium intercalation/deintercalation during the charging/discharging process cause the conductivity between the active materials to degrade or the negative electrode active material to peel off from a negative electrode current collector. In particular, in the case of the negative electrode active materials having a small particle diameter or large specific surface area, side reactions with the electrolyte solution increase as the contact surface with the electrolyte solution increases during the initial lithium intercalation, and consequently, the initial non-reversible capacity increases such that the initial efficiency is disadvantageously reduced.
Therefore, there is a strong demand for the development of a negative electrode active material for a lithium secondary battery capable of overcoming such limitations while also having excellent lifetime properties and excellent high-rate charge/discharge properties.